1. Field of the Invention
The present invention relates to an interface circuit which is capable of protecting a telephone set from transient overvoltage such as surge voltage, and from continuous inflow of overcurrent due to a fault contact between a commercial power line and a pair of subscriber lines.
2. Description of the Related Art
Since the subscriber lines being air-suspended have the possibility of getting transient lightning-induced voltage propagation thereto due to a lightning strike, or receiving continuous inflow of overcurrent for a somewhat long period of time due to a fault contact (or short circuit in connection) with the commercial power line, a protection circuit is often provided in the interface between the telephone set and the subscriber line. As counter measures to possible lightning surges, for example, a structure with a varistor element connected between two subscriber lines, a structure with a varistor element connected between the subscriber line and the grounding wire, etc. are known. When a transient surge voltage exceeding a varistor voltage is applied to the subscriber line, the varistor element will function to protect a speech circuit inside the telephone set by shifting to a conduction mode to absorb the surge voltage.
Moreover, as a counter measure to possible heat generation and fires in the telephone set due to a fault contact between the commercial power line and the subscriber line, for example, a structure having a PTC (positive temperature coefficient) thermistor inserted to the interface between the subscriber line and the telephone set is known. When there is a continuous inflow of overcurrent at the PTC thermistor for a some period of time, an input impedance at the interface will increase along with a rise in the element temperature, whereby the inflow of overcurrent into the telephone set can be prevented.
FIG. 3 is a circuit diagram showing a conventional telephone interface circuit 30. The telephone interface circuit 30 is to perform interface control between a speech circuit 20, which is to process audio signals, and a pair of subscriber lines L1 and L2.
The telephone interface circuit 30 mainly includes a diode bridge 40 which serves to rectify signals traveling inside the pair of the subscriber lines L1 and L2 to supply the signals to the speech circuit 20, a transistor Tr3 which functions as a hook switch for switch-controlling the connection between the pair of the subscriber lines L1 and L2 and the speech circuit 20, a transistor Tr4 which functions as a driver for switch-controlling the on/off state of the transistor Tr3, and a zener diode D5 which serves to absorb possible overvoltage that could be applied to the subscriber lines L1 and L2.
The diode bridge 40 is composed of four diodes D1, D2, D3 and D4.
The transistor Tr3 is to turn on at an off-the-hook state so as to connect the pair of the subscriber lines L1 and L2 to the speech circuit 20, whereas it turns off at an on-the-hook state so as to disconnect the pair of the subscriber lines L1 and L2 from the speech circuit 20.
An emitter terminal E3 of the transistor Tr3 is connected to the subscriber line L1.
A collector terminal C3 of the transistor Tr3 is connected to the speech circuit 20 through a resistor R7.
A base terminal B3 of the transistor Tr3 is connected to a collector terminal C4 of the transistor Tr4 through a resistor R4.
A resistor R3 is connected between the emitter terminal E3 of the transistor Tr3 and the base terminal B3 of the transistor Tr3.
A base terminal B4 of the transistor Tr4 is divided into two lines, one connected to a terminal HC via a resistor R5 and the other connected to the subscriber line L2 via a resistor R6.
The transistor Tr3 is a PNP transistor whereas the transistor Tr4 is an NPN transistor.
The terminal HC is connected to a microcomputer (not shown). At the time when off-the-hook operation, on-the-hook operation, dial pulse transmitting operation or the like is to be carried out, this microcomputer serves to control a voltage V4 at the terminal HC in order to control a base potential of the transistor Tr4.
For instance, in the off-the-hook state, the microcomputer will control the voltage V4 at the terminal HC such that the voltage V4 will be at high voltage. Then the base potential of the transistor Tr4 will rise as an electric potential of the terminal HC rises, whereby the transistor Tr4 will turn on. Then, because a base potential of the transistor Tr3 will drop, the transistor Tr3 will turn on, and thus the pair of the subscriber lines L1 and L2 will be connected to the speech circuit 20.
In the off-the-hook state, in response to a dial input, the microcomputer will control the voltage V4 at the terminal HC. Thereby, the transistor Tr4 will transmit a dial pulse signal.
In the on-the-hook state, the microcomputer will control the voltage V4 at the terminal HC such that the voltage V4 will be at low voltage. Then the base potential of the transistor Tr4 will drop, whereby the transistor Tr4 will be cut off. Then, because the base potential of the transistor Tr3 will rise, the transistor Tr3 will be cut off, and thus the pair of the subscriber lines L1 and L2 will be disconnected from the speech circuit 20.
With respect to the above-described telephone interface circuit 30, however, transistors with high pressure resistance, which are quite expensive, are required to be used as the transistors Tr3 and Tr4 to be connected between the pair of the subscriber lines L1 and L2, and this leads to increase in manufacturing costs.
Moreover, as the above-described telephone interface circuit 30 has to have the resistor R4 inserted in between the two transistors Tr3 and Tr4, the telephone interface circuit 30 is left with little design flexibility.